1. Field of the Invention
The present invention relates to a lens driving apparatus and control method thereof, and an image capturing apparatus and control method thereof and, more particularly, to a lens driving apparatus which corrects a shift of an image plane upon driving of a zoom lens and a control method thereof, and an image capturing apparatus and control method thereof.
2. Description of the Related Art
Some optical apparatuses have a zoom optical system that allows zooming as an imaging optical system. Especially, video cameras normally use a zoom optical system called a rear-focus type. In the zoom optical system of the rear-focus type, an image plane shift (defocus) occurs at the time of zooming by moving a zoom lens. Hence, control is made to maintain an in-focus state by moving a focus correction lens arranged on the image plane side of the zoom lens, so as to correct an image plane shift. A moving position of the focus correction lens is decided in advance for each object distance based on digital cam data (to be simply referred to as cam data hereinafter) indicating a position of the focus correction lens corresponding to a position of the zoom lens.
In order to attain zooming while maintaining an in-focus state in such zoom optical system, the positions of the zoom lens and focus correction lens have to accurately keep a relationship, which is specified by the cam data. The position of the zoom lens can be obtained by counting the number of pulses of a driving pulse signal applied to a stepping motor used to move the zoom lens from a predetermined reference position. For this reason, by controlling the position of the focus correction lens according to the number of pulses of the driving pulse signal (to be referred to as the number of driving pulses hereinafter), the zoom lens and focus correction lens can be moved while keeping the relationship specified by the cam data.
Some optical apparatuses have, as lens driving apparatuses, a driving mechanism configured by a stepping motor as a driving source, a feed screw rotated by the stepping motor, and a rack which is attached to a lens holding member and engages with the feed screw. In order to attain a size reduction of the optical apparatus having such driving mechanism, it is effective to reduce a consumption power by lowering the rotational speed and output torque of the stepping motor, thereby reducing a size of a battery. However, when the output torque of the stepping motor is lowered, if a lens driving load becomes heavier, the stepping motor readily steps out. A driving amount of the stepping motor is normally controlled based on the number of pulses of a driving pulse signal applied to that stepping motor. Hence, when stepping-out occurs, the correspondence relationship between the driving amount of the stepping motor and the lens position is destroyed, and the lens position can no longer be accurately controlled.
Also, it is possible to reduce an engaging pressure of the rack to the feed screw so as to reduce the lens driving load. However, in this case, so-called jumping (the rack overleaps threads of the feed screw) readily occurs, and the lens position can no longer be accurately controlled.
Japanese Patent Laid-Open No. 05-281449 discloses a lens driving apparatus which includes a position sensor used to detect the position of a lens moved by a stepping motor. In this apparatus, when stepping-out of the stepping motor or jumping of a driving mechanism has occurred, the control of the stepping motor is corrected according to the lens position detected by the position sensor, thus allowing accurate lens position control.
Also, Japanese Patent Laid-Open No. 11-110045 discloses a lens driving apparatus which detects the position of a lens moved by a stepping motor by a position sensor, and executes feedback control of the stepping motor using the detection result, so as to improve the lens position control accuracy.
However, an engaging portion of the feed screw and rack, and a coupled portion of the lens holding member used to hold the zoom lens and the rack in the driving mechanism, which transmits the driving force of the stepping motor to the zoom lens, normally suffer backlashes. The feed screw and rack include manufacturing errors such as thread pitch errors. Furthermore, the stepping motor also has an individual difference of an actual driving amount with respect to the number of pulses of a driving pulse signal. Such backlashes, manufacturing errors, and individual difference cause a mismatch between the number of driving pulses applied to the stepping motor and the actual zoom lens position. As a result, the relationship between the positions of the zoom lens and focus correction lens deviates from that specified by the cam data, thus causing a defocus upon zooming.
Furthermore, in order to attain a compact, wide-angle zoom optical system of the rear-focus type, a lens type which drives another correction lens in an optical axis direction in addition to the zoom lens and focus correction lens is available. This lens type is designed, so that zooming is done by moving the zoom lens and the other correction lens which sandwich a stop fixed with respect to the image plane at predetermined ratios, and an image plane shift is corrected by moving the focus correction lens. In this case, an in-focus state cannot be maintained unless the three or more lenses are driven to keep a predetermined positional relationship. Therefore, it becomes further difficult to maintain an object image in an in-focus state upon driving the zoom lens.
However, in Japanese Patent Laid-Open Nos. 05-281449 and 11-110045, by arranging the position sensor for the zoom lens moved by the stepping motor, the position control accuracy of the zoom lens itself is merely improved.